Comparative analysis of the possibilities of transformation of the structure of titanium and zirconium alloys by thermohydrogen treatment is performed. The mechanisms of phase transformation and structure formation in zirconium alloy Zr - 2.5% Nb under reversible alloying with hydrogen are described. Phase diagram of the (alloy Zr - 2.5% Nb) - hydrogen system is plotted using the methods of metallographic and x-ray diffraction analysis and differential scanning calorimetry. The diagram is compared to that of the titanium alloy. The hydride phases in the zirconium alloy are shown to exhibit much higher temperature and concentration (with respect to hydrogen) stability than those of the titanium alloy. It is shown that the original lamellar structure of the zirconium alloy can be globularized by thermohydrogen treatment.